brief knowledge of mRNA structure

1. STRUCTURE OF
MESSENGER RNA (mRNA)
By :
Md Rizwan
Ph.D Dairy Microbiology
2nd Year

2. MESENGER RNA (mRNA)
 Bacteria contains 0.05-0.10 pg of RNA, 6% of its total weight, but
mammalian cells contains 20-30 pg and represents only 1% of its total
weight.
 mRNA comprises only about 4% of the total RNA in the cell but is most
heterogenous type of RNA in cell varying in coding region.
 Messenger RNA is a single stranded RNA.
 The mRNA carries genetic information from DNA to Ribosome present in
cytosol, where it is used as a template for protein synthesis.
 Messenger RNA is furnished with genetic code (Message ) as
sequence of codons .
 Each codon consists of triplet of bases .
 Genetic code is Universal, Comma less, non overlapping , degenerate and
Non ambiguous.
 The message of mRNA is read consecutively in 5’→3’ direction. Each amino
Acid is recognised by specific codon.

3.  Most mRNA in mammalian cells are very stable.
 Exist in cytoplasm as Ribonucleo-protein particles (RNPs), that protects mRNA from
digestion by nucleases.
 mRNA has a 5’ non coding sequence (5’NCS), a coding region and a 3’ NCS.
 All are capped at 5’ end and have polyadenylate sequence at 3’ end that protects the
mRNA against exonuclease attack.
 Stem loop structure in the 5’ and 3’ NCS, features in the coding sequence, and the
AU-rich region in the 3’ NCS thought to play role in mRNA stability.
 The poly-A tail must be shortened to about 10 ntds before mRNA can be degraded.

5.  mRNA is short lived between several seconds to few minutes in bacteria but longer
lived in eukaryotes possibly because it has to move out of nucleus to the site of
protein synthesis the ribosome present in the cytoplasm
 In Prokaryotic organisms ribosome can attach to mRNA, while it is being transcribed.
In this situation translation begins at the 5’ end of mRNA while the 3’ end is still
attached to DNA
 In prokaryotes transcription and translation are coupled. Prokaryotic mRNA
undergoes very little processing after transcription.
 Prokaryotic mRNA has at its 5’end has a Shine Dalgarno Sequence which is rich in
purine nucleotides and helps binding of mRNA to 30 S Subunit of Ribosome by
establishing H bonds with pyrimidine nucleotide sequence present on the 3’ end of
single mRNA ( 16 S ) of smaller subunit.
PROKARYOTIC MESSENGER RNA

6. Features of Prokaryotic mRNA

7.  Eukaryotic mRNA arises from Post transcriptional modification of large RNA precursors,
about 5000 to 50000 ntds long, called heterogenous nuclear RNA (hnRNA).
 A cap of ATP/GTP is added at the 5’ end after transcription. G or A get methylated for it.
Cap facilitates binding of mRNA with ribosome and provide stability
 Cap is followed by Non coding (UTR) region I ,which is used for binding of mRNA with
smaller sub unit through its single rRNA.
 Initiator codon AUG follows UTR I which directs to introduce Methionine at first position.
 It is followed by coding region (1200-1500 nucleotides) and can code for polymerisation of
400 to 500 AAs.
 On the 3’end A poly Adenine (250 ntds) tail is added after transcription which also
provides stability to mRNA
 Precursor Eukaryotic mRNA possess both coding (Exons) and Non coding (Introns)
regions and require some processing to give mature functional form.
 In case where introns are present, by splicing introns are removed and exons in sequence
are ligated to form continuous stretch of coding region.
EUKARYOTIC MESSENGER RNA

9. THANK YOU
References:
1: Prescott, Microbiology ,7th edition, page -272.
2: T.A Brown, Genomes 4, 4th edition, Page -13-14.